This invention relates to an improved capacitance humidity sensing element for use in measuring the humidity of the atmosphere and is an improvement on U.S. Pat. No. 4,429,343, issued Jan. 31, 1984 to one of the present coinventors. This patent is hereby incorporated by reference as a part of this specification.
In the referenced patent there is described a humidity sensing element which functions by sensing changes in the dielectric constant of a thin water absorbing polymer film as induced by changes in the relative humidity of the surrounding atmosphere. As set forth in that patent, the interdigitated electrode structure underlying the polymer film is capacitively coupled through the film. Thus, the capacitance between electrodes is dependent upon the dielectric constant of the polymer film and the concentration of water molecules absorbed in the film.
The patent discloses that by using a polymer film whose thickness is greater than the period of the fingers of the interdigitated electrodes, it is possible to insure that the capacitance between the fingers will not depend upon the conditions outside of the polymer film or at its surface. This property is very important for practical devices, because it insures that the device will be independent of surface contamination or extraneous coatings on the polymer film.
In practice the minimum spacing possible between the interdigitated electrodes, in the structure of the referenced patent, is limited by the patterning techniques available. Thus, as taught by the patent, the minimum spacing between the electrodes will dictate what the minimum thickness for the polymer film can be, while still maintaining the desired independence from surface contamination. That minimum will then dictate the minimum response time for which the sensing element can be designed, for a decreased response time is obtained by decreasing the polymer thickness.
While the above mentioned patent suggests polymer thicknesses on the order of 50 microns, U.S. Pat. No. 4,164,868, issued to Suntola on Aug. 21, 1979, describes elements using a polymer film of 1 micron thickness, or less, over a two electrode arrangement, to obtain a faster response. As described in the patent, the polymer is overlayed by a thin electrically conductive water permeable layer. According to the patent, the effect of the overlay is to make the equivalent circuit between the two electrodes appear as two series capacitors in which the polymer is the dielectric and the thin conductive layer acts as the common electrode. To be complete, it would be appropriate to include in the equivalent circuit a resistance between the two capacitors in order to represent the effective resistance of the thin conductive layer. In addition, it should be noted that, the Suntola patent indicates that the thickness of the conductive layer is 0.1 to 1 microns, which would be generally impervious to water except through fractures which might be present. No method is described for making the Suntola device water permeable. If, by some means openings are provided to make the conductive layer highly permeable to water, then the conductive layer will not protect the device from the influence of contaminants on the surface, for the electrical field between the electrodes will fringe out through the conductive layer at the openings. Also, the unprotected conductive layer will be subject to the corrosive effects of atmospheric pollutants.
Other problems are presented by the thin conductive layer of the Suntola patent. The presence of that layer will tend to limit the response time of the element, for it will tend to limit the diffusion of water molecules through the conductive layer. Since metal films have very low diffusion rates, the film will have to be extremely thin. A practical limit, however, is reached in such devices when the sheet resistance of the conductive film gets to a magnitude such that the effective resistance between the two series capacitances is comparable to the impedance of those capacitances, thus causing difficulty with the capacitance measurement itself as might result from temperature changes affecting the resistance of the thin conductive layer.
It is an object of this invention to provide a structure for a humidity sensing element and a method for producing that element which will allow a minimizing of the response time while maintaining the element immune to the influences of surface contaminants.
It is a further object of this invention to provide a structure which uses an overlayed conductive film to make possible a minimization of the dielectric layer thickness to thereby minimize the response time, but which also eliminates the influence of that film on the response time of the element and maintains the response time of the element as solely a function of the thickness of the dielectric layers which cover the electrodes.
It is a further object of this invention to provide a structure which uses an overlayed conductive film to make possible a minimization of the dielectric layer thickness and thereby minimize the response time, but which also makes the response of the element independent of difficulties due to the effect of the sheet resistance of the conductive film on measurements made with the element.